Oxidative Degradation (Most common)

Oxidative degradation occurs when oxygen in the air reacts with the fluid through a free radical mechanism, forming larger molecules that eventually become polymers or solids. These byproducts thicken the fluid, raising its viscosity. A thicker fluid is harder to pump, transfers heat less efficiently, and increases the risk of coke deposits forming in the system. Oxidation also raises the Total Acid Number (TAN), further accelerating fluid degradation.

Like most chemical reactions, oxidation speeds up as temperature increases. At room temperature it is barely measurable, but at elevated temperatures the rate grows exponentially. Systems that lack oxidation-reducing measures—such as nitrogen blanketing of the expansion tank—are especially vulnerable.

In layman’s terms: oxidation happens when hot fluid is exposed to air. The most visible sign is sludge build-up, especially in low-flow areas like reservoirs or expansion tanks. TAN is the standard measure for oxidative degradation, and rising TAN values indicate worsening oxidation. Once the TAN climbs above 1.0–1.2 mg KOH/g, it usually signals trouble. In smaller systems with poor drainage, acidic residues left behind can quickly contaminate new fluid after a refill. That’s why ensuring maximum evacuation of spent fluid—particularly when TAN is above 1.0—is essential before refilling.

Thermal Degradation

Thermal degradation, or thermal cracking, occurs when the fluid is heated beyond its recommended maximum bulk or film temperature. This excess heat breaks carbon–carbon bonds within the fluid molecules. The reaction may stop there, producing smaller molecules, or continue with fragments recombining into larger polymeric molecules. In heat transfer terminology, these are known as “low boilers” and “high boilers.”

Low Boilers

Low boilers show up as a measurable drop in flash point and viscosity, along with an increase in vapor pressure. Higher vapor pressure disrupts system efficiency, can cause pump cavitation, and lead to premature pump failure. A reduced flash point also creates significant safety and operating risks.

High Boilers

At extreme temperatures—typically above 400°C (752°F)—thermal cracking becomes more severe. Not only do carbon–carbon bonds break, but hydrogen atoms separate from carbon atoms, leading to coke formation. High boilers increase viscosity as long as they remain in solution. Once solubility limits are exceeded, they form solids that quickly foul heat transfer surfaces. In these cases, fouling occurs rapidly and can shut down the system entirely.

In layman’s terms: thermal degradation is caused by overheating the fluid past its boiling point. Like water, the fluid boils and produces lighter vapors. Overheating reduces viscosity and lowers key safety thresholds, including flash point, fire point, and autoignition temperature. Excessive cracking can quickly push these limits to unsafe levels.

Flash Point

Flash point is a critical safety indicator, but it is not usually a concern unless it drops significantly from the fluid’s original specification. In fact, most heat transfer systems operate at temperatures above the flash point without issue. Problems arise only when the flash point decreases dramatically due to degradation.